Vehicle anti-skid brake systems, or computer brake systems or anti-wheel-lock systems as they are sometimes referred to, include a control feature which, during operation in its normal mode, prevents the brakes from locking the vehicle wheels when the driver applies high foot pressure to the brake pedal of the vehicle. This function is accomplished, broadly, by causing the braking force to release just before a locked wheel condition occurs and to rapidly reapply braking force as soon as the potential of a locked condition disappears, even though the driver's foot continues to apply high pressure to the brake pedal throughout. The cycle of release and reapplication is very rapid, perhaps 300 to 500 milliseconds, and the cycle will normally be repeated a number of times while the vehicle is being decelerated. In general these brake systems include a sensor for sensing the speed of a vehicle wheel and for generating an electrical signal at least in part from the sensed wheel speed, the signal having a predetermined characteristic which varies with the wheel speed. When the signal indicates that the deceleration of the wheel is so great that locking of the wheel is imminent, a control circuit of the system operates to reduce the force which frictionally engages the elements of the brakes. Since at this time the vehicle is moving faster than the wheel, the wheel will now begin to accelerate. Many control systems sense this condition and cause the brake system to immediately reapply braking force. If the condition is not sensed a fail-safe circuit renders the control system inoperative.
The principles on which the above-summarized operational characteristics of anti-skid brake systems are based are well understood and accepted. It has long been realized, of course, that locked wheels on a moving vehicle do not decelerate the vehicle rapidly since the sliding friction between pavement and non-rotating tires does not absorb kinetic energy at a sufficiently high rate. It is generally accepted that deceleration of a vehicle is maximum when the linear speed of the vehicle wheels is 10% to 30% less than the speed of the vehicle, that is when the slip ratio lies between 10% and 30%, slip ratio being defined as (VS-WS)/VS .times. 100 where VS is vehicle speed and WS is wheel speed. When a wheel is braked to the extent that its speed is more than 30% less than the vehicle speed, the sliding friction developed between tire and ground surface reduces traction to such an extent that the brakes inherently cause the wheel to lock. Therefore the broad object of any anti-skid brake system is to prevent too rapid deceleration of the wheels when the brakes are applied. In practice it is generally agreed that a wheel deceleration of more than about 1.25 g (g being the acceleration of gravity) leads rapidly to wheel lock-up and therefore many anti-skid systems employ this deceleration value in the generation of the brake-withdrawal signal.
The present invention is concerned with in-place testing of anti-skid brake systems installed in a vehicle under dynamic conditions to determine whether all sensors in a given anti-skid brake system are operative. For example, for a system which includes a sensor associated with each of two opposite rear wheels of a vehicle and a control circuit which in its normal mode of operation releases brake force to both wheels in response to either sensor's sensing of a potential lock-up of its respective wheel, the present invention provides a rapid and reliable technique for determining whether each sensor does in fact actuate the control circuit independently of the other sensor. The test in effect simulates the dynamic conditions which would be present in the event that one rear wheel of the vehicle was in contact with a patch of ice or other slick spot while the brakes were being applied. In this situation the wheel in contact with the slick spot would tend to lock up even with only a small brake force being applied. In the normal mode of operation of the brake system the sensor associated with that wheel would actuate the control circuit to release brake force to both wheels. The test contemplated by the present invention simulates this condition sequentially for each wheel and indicates in each case whether the sensor and control circuit have operated in the normal mode.
A dynamometer modified and operated according to the invention is a suitable machine for carrying out the test. By employing the testing technique of the present invention a vehicle manufacturer can readily obtain, in a matter of minutes, a permanent visual test record for each vehicle establishing that the vehicle was delivered to the dealer with an anti-skid brake system in proper working order. This is highly desirable from the manufacturer's viewpoint because it establishes that the manufacturer has complied with whatever safety standards are required by law. The visual record may therefore be of considerable significance in placing liability in the event that the allegation is made later that an accident was caused by a faulty brake system. The same advantage is available to the dealer if he continues the record of a given vehicle by testing it when it is delivered to the customer. Similarly operators of fleets of trucks and buses can continue the record by including the test in their normal periodic safety inspections.
The testing procedure of the invention described in the aforementioned U.S. Pat. No. 3,979,950 makes use of the fact that an anti-skid brake control system during operation in its normal mode causes the vehicle wheels to slow down stepwise, as discussed briefly above. These alternate decelerations and accelerations of the wheel can be sensed by a dynamometer in any of several different modes of operation, when certain special controlled conditions are pressent, and the resulting electrical signals can be either preserved in a computer memory for subsequent printout or immediately converted to some convenient visual record. The instrumentation must of course accurately measure changes in the electrical signals occurring within very short time intervals, because anti-skid controls are capable of releasing and reapplying braking forces very rapidly. The electrical signals may be representative of any of the dynamic characteristics conventionally measured by a dynamometer, such as torque, wheel speed, wheel acceleration and deceleration, horsepower of balance (brake equalization). In all cases the electrical signal utilized by the present invention will vary with a change in the speed of the vehicle wheel. Either idler roll speed alone or drive roll alone or both in combination may be sensed directly for purposes of generating a signal indicative of speed, speed change, balance or horsepower. Even if the signal does not include a direct measurement of the speed of a roll the signal will still vary with vehicle wheel speed, as for example when a simple direct torque signal is employed, due to the operation of the anti-skid brake system.
The present invention, in determining whether a control circuit designed to be operated by either of two sensors is actually operated by each sensor independently of the other sensor, needs to measure or sense only the initial release of the brakes.